The invention relates to image forming systems, and more particularly relates to suppressing visual defects associated with multiple swath printing.
There are a number of different image forming systems in use today for generating images on a print medium. One such image forming system is an Acoustic Ink Printing (AIP) system. The AIP system employs focused acoustic energy to eject droplets of ink from a printhead onto a printing medium. Printheads utilized in AIP systems most often include a plurality of droplet ejectors, each of which emits a converging acoustic beam into a pool of fluid, such as ink. The converging acoustic beam focuses at the border between the ink and the air. The modulation of the radiation pressure exerted by the beam of each print ejector against the surface of the ink selectively ejects droplets of ink from the surface. Different combinations and quantities of inks and ink droplets can combine to achieve a wide variety of colors and shades demanded for the creation of documents.
Different inks and ink droplets combine in AIP, and other systems, to form a series of printing swaths, which proceed across a printing medium to form a desired document image. The alignment of abutting regions of the printing swaths has an impact on print quality. Such regions can occur in either reciprocating carriage printers when the printhead is advanced, or from the alignment of the boundary between two same-color printheads. The alignment problem compounds for multiple small-drop printing as found in AIP systems. Swath alignment defects in printed images take the form of horizontal edges, which are particularly noticeable.
One approach includes a system in which the ends of the swaths are randomly placed as disclosed in a separate application entitled xe2x80x9cA METHOD AND APPARATUS FOR STITCHING PRINT SWATHS IN AN IMAGE-RENDERING DEVICExe2x80x9d by Ellson, et al., filed Dec. 18, 2000 with the USPTO (application Ser. No. 09/739,854) hereby incorporated herein by reference. The teachings of the present invention can adjust the ends of the offset rows in the manner disclosed below.
Another known approach is to provide no form of stitching at the swath boundaries. The approach works well if the advance between swaths is small and fairly accurate. Small advances can be done quickly, accurately, and any defects will occur at a higher frequency. However, when larger advances are needed as in the case of high speed printing with large printheads, or swaths are printed by separate printheads or elements, accurate and efficient placement of the swaths is difficult.
There exists in the art, for the foregoing reasons, a need for a method of printing in an image forming system that suppresses defects in the printed images. The present invention is directed toward further solutions in this art.
An image forming system in accordance with one example embodiment of the present invention includes a method for printing on a printing medium, such as, e.g., a paper sheet. The method includes the steps of determining a collection of image data lines representing an image to be printed. Each image data line contains data for printing the line, in predetermined line increments. The image forming system then prints a first swath comprised of a collection of data lines, the specific number depending at least partially on the width of the swath. The first swath includes a first portion of a stitch line that disposed along one edge of the first swath. The edge, for illustrative purposes, is typically a bottom most edge of the swath on the printing medium, or the last data line printed in the swath. The printing medium advances, after completion of the first swath, a predetermined distance to prepare for the printing of an additional swath. The image forming system prints a second swath, which includes a second portion of the stitch line disposed along one edge of the second swath. The edge of the second swath is typically the upper most edge of the swath, or the first data line printed. The same image data line provides the instructions to print both the first and second portions of the stitch line, between each swath.
The method, according to one aspect of the invention, further includes the step of measuring the amount of advance subsequent to each advancing step. The middle portion of printhead ejectors are used for printing for an accurate advance. The outer ejectors are reserved for occasional use to reach distal edges of a swath to correct for large errors in the swath advance. In the case of a large advance error, the swath is printed with ejectors that lie nearest to the intended pixel locations of the swath. This has the effect of limiting the effective advance error to half the pixel resolution.
The method, according to another aspect of the invention, further includes the step factoring the measurement values into the further calculation of the number of drops used in the second portion of the stitch line printed in the second swath.
The first portion of the stitch line, according to yet another aspect of the present invention, ranges between 0 and 100% of the final stitch line. The second portion of the stitch line, in such an instance, prints in an amount of 100% minus the percentage printed in the first portion of the stitch line. The varying percentage portions of the stitch line printed in each of the first and second swaths compensate for advance defects.
The step of printing the first swath includes printing a first portion of the stitch line representing 50% of the stitch line according to still another aspect of the present invention. The second portion of the stitch line, printed in the second swath, then comprises between 0 and 100% of a nominal definition of the stitch line.
A similar procedure is used for printing with multiple printheads or elements, except in such a case the alignment of the printheads is measured in advance. The actual alignment at the time of printing can vary in ways that can be predicted. One variable that can affect the alignment is temperature. If alignment error is known prior to printing either swath, there is greater flexibility in the determination of the percentage of drops printed for each portion of the stitch. The result is a greater range in the amount of ink that can be deposited from a minimum of 0% to a maximum of 200% totaled between the two stitch portions.